Local telephone companies have used many different systems to provide Plain Old Telephone System (POTS) services to subscribers in rural areas. For a selected POTS application, a particular system may be implemented because it can overcome the limitations or problems that are commonly associated with rural telephone service delivery. Typical problems associated with the delivery of telephony and data services to a small number of subscribers in a rural location include a shortage of available copper wire pairs for carrying signals, an extended distance between the Central Office (CO) and the subscriber, a lack of available electrical power to operate components of the telephony system, and certain communication technology limitations. Representative systems for addressing at least some of these problems include an analog "direct connection" system, an analog carrier system, a universal digital loop carrier (UDLC) system, an integrated digital loop carrier (IDLC) system, and an add-drop multiplexing system. These prior telephony systems will be described in more detail below in connection with FIGS. 1, 2, 3 and 4.
Direct Connection System
FIG. 1 illustrates an analog direct connection system 1 for connecting a subscriber's telephone 5 to a CO switch 3 via a local loop 2 comprising copper wire pairs. This direct connection system can be implemented for a rural application unless the existing set of copper wire pairs for carrying telephony signals is smaller than the numbers of subscribers to the telephony service. In the event that the local loop 2 extends for a distance greater than approximately 40 kft., the CO switch 3 is typically equipped with battery voltages in excess of the nominal -48Vdc, and amplifiers 4 can be used within the local loop 2 to increase the level of the telephony signal. The direct connection system is not appropriate for applications involving a distance between the CO switch and the subscriber's telephone that is outside the limits of switch capability.
In contrast to the direct connection system described above, multiplex systems can be used to transport two or more telephone conversations or data links between a subscriber's premises and a telephone terminal or CO switch. As described below in connection with FIGS. 2, 3 and 4, prior multiplexing systems have used a variety of transmission techniques and multiplexing and encoding schemes, and delivered a variety of telephony services.
Amplitude Modulated Telephone Carrier System
U.S. Pat. No. 4,087,639 describes a representative amplitude modulated telephone carrier system that supports the delivery of POTS for both single and multiple party lines over a single pair of copper wires. The system, which is powered from the CO, modulates an analog speech signal onto a single carrier in the very low frequency (VLF) or low frequency (LF) bands to complete a telephony transmission. Subscriber terminals can be placed at any location along the telephone line by using a bridged tap connection. Carrier signals can be regenerated at prescribed intervals along the telephone line by using bidirectional amplifiers.
Referring now to FIG. 2, which illustrates a typical amplitude modulated telephone carrier system 6, a multiband analog double sideband modulated carrier is carried on a single pair line 7. Subscriber drops 8 can be placed at any location along the line 7 between a Central Office Terminal (COT) 10 and a termination network 11. The subscriber's telephone 5 is connected to one of the subscriber drops 8 via a wire pair 9. Each subscriber drop 8 is powered from the line 7 and is connected to that line by a bridged tap splice in a conventional "T" connection. A bidirectional repeater 12 can be inserted along the line 7 to amplify the carrier signals on an as needed basis. The COT 10 is located at the CO 3 and is typically powered by a local -48V dc source via an interface cable 13. The COT 10 can house plug-in cards 18 for converting the analog carrier on the line 7 to a conventional 2-wire POTS signal for distribution to the CO 3 via an interface 19.
For a typical amplitude modulated telephone carrier system, the maximum distance between the CO switch and a subscriber's telephone can exceed 100 kft. In contrast, the distance between subscriber drops and the subscriber premise associated bridge-tap connections is commonly two or three kft. However, the distance `d` between any two bridge tap connections, as shown in FIG. 2, can range from 0 feet to approximately 50 kft.
The amplitude modulated telephone carrier system can support limited data services, but cannot deliver these services by means other than low speed (nominally 4800 bps) analog modems. Furthermore, this analog-type carrier system cannot support the delivery of CLASS features, such as caller identification, call forwarding and call waiting, to subscribers. This effectively limits the growth of possible services that can be offered by a telephone company that relies on this analog transmission system. The limited data services and the absence of CLASS functions are a direct result of the analog component implementation of the amplitude modulated telephone carrier system. In addition, the telephone company must maintain the signal characteristics of an amplitude modulated telephone carrier system, including impedance levels, current levels, and voltage levels, because of the analog implementation of this system. For example, this system requires a terminating impedance at the end of the subscriber loop.
Universal Digital Loop Carrier System
A typical universal digital loop carrier system, also described as a UDLC system, uses a digital multiplexer, placed between the subscriber's telephone and the CO, to combine subscriber channels into a single high speed digital line signal for transmission to a de-multiplexer or a COT. Because a multiplexed digital line cannot carry signals as far as a corresponding analog line, the digital line often requires a number of digital repeaters to boost signal level. A typical digital multiplex system will carry from 24 to 3000 POTS circuits.
FIG. 3 illustrates the components of a typical UDLC system 16, which represents a point-to-point multiplex system. A remote terminal (RT) 17, connected to voice and data channels, places voice and data signals in a digital format and time-division multiplexes these digital signals onto a single digital carrier for transmission. The digital carrier is transmitted by a carrier line 18 extending between the RT 17 and a COT 19. One or more bidirectional repeaters 20 can be placed in the line 18 to boost the level of the digital carrier. The line 18 is terminated at the COT 19, which performs the inverse function of the RT 17. In particular, the COT 19 maps individual channels on a one-for-one basis and reproduces the channels in original form. Channel grooming and interchange, however, are not performed by the COT 19. The number of channels supported by the RT 17 is the same as that of the corresponding COT 19.
The customer premises equipment (CPE) 5 is connected to a POTS line card 21 in the RT 17 via a copper local loop 22 having a length of typically less than 12 kft. The RT 17 is typically powered by a local AC source via an interface cable 23a. A channel card 24 of the COT 19 is connected to the CO 3 via a copper pair 25. The COT 19 is typically powered from a local -48Vdc power supply, typically located at the CO 3, via an interface cable 19a.
Integrated Digital Loop Carrier System
Although IDLC systems have evolved from the UDLC system design shown in FIG. 3, IDLC systems typically possess a higher level of control over the voice or data channels. In contrast to UDLC systems, IDLC systems typically can assign channels to time slots (DS-0s), concentrate lines into fewer trunks, and reformat dialing and other signaling information. A significant difference between the UDLC and the IDLC systems is the elimination of the COT in the IDLC system. In particular, the line carrying digital signals is directly connected to the CO switch via an appropriate interface for carrying a multiplexed stream of digitized voice signals.
A fundamental limitation of both the UDLC system and the IDLC system is the use of a remote terminal powered by local AC power source rather than by power generated at the CO (or COT) and carried to remote components via the carrier line. In addition, these prior systems rely upon a single RT to distribute telephony signals to subscribers. This signal distribution implementation limits the use of the UDLC and IDLC systems to a single location having a relatively large concentration of subscribers, wherein spacing between these subscribers is limited. Consequently, UDLC and IDLC systems are typically installed in a multiple-home subdivision or business park representing a concentrated subscriber base.
Add-Drop Multiplexer System
An add/drop multiplexer system 30, which is shown in FIG. 4, uses remote terminals on an optical fiber ring to transport signals between a CO and subscribers. Remote add/drop terminals (ADT) 26A-26C are connected to each other and to the COT 27 via one or two fiber optic cables 28. Because ADTs can be located in either a telephone company-owned cabinet or in a subscriber's premises, a power interface cable can be connected to either a local power source, typically 120 Vac or -48 Vdc, to power each ADT 26. The add/drop multiplexer system also uses a COT 27 with capabilities similar to that of the UDLC system illustrated in FIG. 3. A significant difference, however, is the use of a pair of optical interfaces rather than a single electrical interface, such as copper wire pairs, for connecting signals between the ADTs 26 and the COT 27. For example, two optical fiber cables 28 are connected between the COT 27 and the ADTs 26. The fiber optic cables 28 operate in a simplex mode where one cable is used for transmit operations and the remaining cable is used for receive operations. In this way, signal traffic rotates around the fiber ring formed by the four segments of the fibers 28 in either a clockwise or counter-clockwise direction. Telephone hand sets 5a and 5b are respectively connected to an ADT 26A and an ADT 26B via line cards 26' and a copper pair 28'.
To complete a call from the telephone 5a, the ADT 26A multiplexes a DS-0 signal into a digital stream and sends the digital data via the transmit fiber to the ADT 26B, where the data stream is selectively demultiplexed and traffic destined for that terminal is delivered to appropriate subscribers. The DS-0 traffic from the telephone 5a remains in the digital stream and is transmitted to an ADT 26C, where similar demultiplexing occurs. The DS-0 traffic from the telephone 5a again remains in the multiplexed stream and is transmitted to the COT 27. At the COT 27, the DS-0 traffic from the telephone finally demultiplexed and sent to the CO 3 via a copper pair 29 and an interface card 27b. Similarly, traffic from the CO 3 is routed to the telephone 5a and to other subscribers via the ADTs 26A, 26B, and 26C. The advantages of the add/drop multiplexer architecture are that time slots can be reused within a ring and that traffic can be sent between ADTs 26. For example, the telephone 5a can be connected to the telephone 5b without traffic passing through the CO 3, thereby conserving system bandwidth.
Although the prior telephony systems described above have addressed certain problems faced by telephony subscribers, these systems do not solve the problem of providing combined voice and data telephony services for rural telephony subscribers by communicating a digital carrier signal over an existing single wire pair cable. A typical rural telephony application is directed to a population density of less than 100 persons per square mile, wherein the subscribers are typically served by telephone loops comprising copper wire pairs extending up to 50 kft. Prior systems are not easily adaptable to support the addition of new subscribers in rural areas, particularly those rural locations having service needs increasing at a rate of five to ten percent per year. Many existing rural telephone installations have fewer copper wire pairs than the number of existing telephony subscribers, thereby limiting the amount of subscriber growth that can occur for any particular rural location. Although a basic solution to the limited installation of wire pairs is simply to install new copper cables from the CO to the subscribers, this solution is both time consuming and expensive for telephone companies because the cost of obtaining and laying cable alone can far outweigh the cost of the supporting electronics installation.
In view of the foregoing, there is a need for an alternative telephony system to service the communications needs of subscribers in rural locations. In particular, there is a need for a telephony system that can utilize an existing copper wire pair installation to carry digital telephony signals while replacing electronic components to provide a level of telephony services presently available only in urban locations. This replacement system should provide subscribers with a higher level of POTS services, while allowing telephone companies to offer advanced voice and data services and to increase the reliability of the overall telephone network. The replacement solution is preferably a digital implementation for carrying voice and data signals throughout the local loop extending between the CO and the subscriber's telephone. A digital implementation can provide superior transmission performance when compared to its analog counterpart because the digital implementation can support an increased quality of telephony service by limiting analog noise and cross-talk signals. The present invention addresses the above-described issues by providing a replacement-type telephony system for rural applications based on a digital carrier system architecture for communicating digital carrier signals on a single wire pair cable.